This invention relates to processes for crosslinking compositions containing the polymeric corn protein zein, thus increasing the strength and stability of articles made from such compositions.
Zein is a naturally occurring protein polymer, obtained as a product of industrial corn processing. Compared to most proteins, zein is characterized by a relative deficiency of hydrophilic groups. In zein, the high proportion of nonpolar and amide side chains accounts for the solubility of zein in organic solvents and its classification as a prolamine.
Zein is a globular protein in its native state. Most processes to make zein fibers involve, first hydrating the protein, realigning it as desired (e.g., by spinning), and finally stabilizing the new alignment by crosslinking. These processes most often start with an alkaline zein solution which is wet spun into acidic coagulating baths. After spinning, crosslinking can be induced by treatment in coagulating baths with formaldehyde, and subsequent further hardening or stabilizing treatments, typically using formaldehyde, followed by drawing stages to improve tenacity.
Croston et al., describe such a process in "Zein Fibers . . . Preparation by Wet Spinning", Industrial and Engineering Chemistry, 37 (1945) 1194-1198. Croston et al. call for a precuring bath of formaldehyde after fiber spinning, which was found to be necessary prior to the final stretching of the fiber tow.
Zein's chemical inertness and globular structure make molding articles difficult. U.S. Pat. No. 3,497,369, discloses a composition of zein, which is substantially dry and which upon the addition of warm water, forms a pliable plastic composition which may be pulled like taffy, molded, or worked with as modeling clay. The composition consists of zein and a small portion of a plasticizer such as glyceryl monoricinoleate. About 5 parts of plasticizer for every 75 parts of zein is most preferred.
This reference describes a process for making bristles from zein, wherein extruded zein fibers are cut into suitable lengths, treated with a chemical hardening agent, such as formaldehyde, and subsequently stretched and subjected to a second hardening treatment.
The environmental implications of using aldehydes as crosslinkers for zein, or of adding plasticizers to zein compositions, make such processes commercially undesirable today. The expense of treatment and disposal make production of useful zein fibers or articles unattractive. Thus, there is a need for environmentally friendly stabilizing agents for zein, to be used in production of zein fibers or articles. The present invention addresses such a need.